1. Field of the Invention
The present invention relates in general to charge storage devices such as supercapacitors and batteries, and more particularly to nanostructure-films deposited on substrates for use in charge storage applications.
2. Description of Related Art
The contents of all references, including articles, published patent applications and patents referred to anywhere in this specification are hereby incorporated by reference.
Some charge storage devices such as supercapacitors and batteries require high surface area materials that form a double layer with an electrolyte. Different forms of carbonaceous materials such as carbon black (M. Min et al J. Electrochem. Soc 153, A334 (2006), S-K. Kuo and L-L. Wu J. Power Sources 162,1437 (2006)) and carbon Nanotubes (CNT G. H. Deng et al Carbon 43, 1557 (2005), G-X Wang et al Solid State ionics 176, 1169 (2005)) have been used as electrode materials. The high surface area materials are used as the electrode, with the charge collectors usually metals. Such configurations can have several disadvantages, especially for applications where flexibility, light weight and thin films are required. Because of the metal charge collectors, simple, room temperature fabrication processes are not applicable. At the same time, cost and weight associated with the metal parts may be an issue.
As described herein, the term “electrode” refers to the portion of an energy storage device that physically interacts with an electrolyte to store charge (e.g., in a polarized double layer in a supercapacitor, in a chemical reaction in a battery, etc.). As described herein, the term “charge collector” refers to the electrically conductive element of an energy storage device that is in substantial physical and electrical contact with an electrode surface, and which serves to transfer charge from the electrode to an electrical contact pad (e.g., in an energy storage device wherein an electrode and a charge collector form thin films, each having essentially two planar surfaces, one of the electrode surfaces is generally in physical contact with an electrolyte while the other electrode surface is in physical and electrical contact with a charge collector surface; the contacting electrode and charge collector surfaces generally share similar perimeter dimensions). Note: as used herein, a charge collector can be distinguished from an electrical contact pad in that an electrical contact pad has a small surface perimeter relative to that of a charge collector with which it is in contact (e.g., whereas the thin-film charge collector described above has a surface perimeter similar to that of the thin-film electrode with which it is in physical and electrical contact, the perimeter of a corresponding electrical contact pad connected to the charge collector will be much smaller (e.g., less than ˜40% of the surface perimeter of the charge collector).
Both batteries and supercapacitors are charge storage devices. While supercapacitors store the electrical energy in polarized double layers along the electrodes surfaces, batteries derive their energy from chemical reactions in the active materials. A battery typically consists of one or more cells which are in turn made up from two electrodes (anode and cathode) an electrolyte and a porous separator.
Batteries are classified into two groups, called primary and secondary batteries. Primary batteries can only be used once and are not rechargeable. Because of their short lifespan the materials used in these cells need to be cheap and environmental friendly. Several patents (U.S. Pat. No. 6,838,209 “Flexible thin battery and method of manufacturing same”; U.S. Pat. No. 6,858,349 “Battery cathode: carbon fibers”; U.S. Pat. No. 5,747,190 “Multilayered battery having a cured conductive ink layer”) describe such devices.
In many instances, carbon black and/or metals are being used as a material for current collectors and/or to increase the conductivity of active materials (MnO2/carbon black paste) and as active electrode material (Li-ion batteries) in current battery designs. The disadvantage of using carbon black for these tasks is its relatively poor electrical conductivity (as compared to metals or carbon nanotubes), which requires large amounts of this material to be used. Metal current collectors, while providing good conductivity, have the disadvantage of having a large mass. This, in turn, decreases the possible energy densities (per kg or per 1).
Batteries have, in addition to an electrolyte and separator, two components: an anode and a cathode. As an example, MnO2 together with an electrically conducting medium such as carbon black is used as an anode, and for a cathode, a film of Zn is used. The configuration can not be fabricated by a simple, solution based deposition process, the process required for the majority of applications.
There is thus a need for a charge storage device that is cheap, has appropriate performance and can be disposed without creating environmental hazards. An embodiment of the present invention includes a device that can use carbon nanotubes (CNTs) to function as both an electrode and a charge collector.